Conventional steel reinforcement includes solid solution reinforcement, reinforcement with second phases with martensite or the like, precipitation reinforcement, and formation of fine grains. Above all, the method of forming fine grains in steel is the best for increasing both the strength and the toughness of steel and form improving the strength-ductility balance in steel. This method does not require any expensive elements such as Ni, Cr or the like, and it is said that high-strength steel can be produced according to the method at low production costs. From the viewpoint of forming fine grains in steel, it is expected that when the size of fine ferrite grains constituting steel could be reduced to 3 .mu.m or smaller, the strength of the steel could be greatly increased.
At present, however, it is impossible to much more increase the strength of steel obtainable in the current ordinary working and heat-treatment, in which the grains have a size of around 5 .mu.m, or so, even though the steel of that type could have relatively high strength.
Steel comprising finer ferrite grains could have higher yield strength and higher tensile strength, but is problematic in that its uniform elongation is greatly lowered and that the increase in its yield strength is larger than that in its tensile strength. In other words, the yield ratio of the steel is high. This means the decrease in the n value (the work-hardening coefficient) of the steel. The same shall apply to ultra-fine, single ferrite phase steel having a ferrite grain size of not larger than 4 .mu.m. That is, the strength of the steel could be increased but the elongation is greatly lowered.
Given that situation, it has heretofore been said that, in order to increase the strength of ferrite steel and to improve the strength-ductility balance thereof, needed is any other technique that is quite different from the conventional technique for much more fining the ferrite grains constituting the steel.
Heretofore, the conventional controlled rolling-accelerated cooling technique has been one effective means for forming fine ferrite grains that contribute to the increase in the strength of low-alloy steel. According to the technique, both the cumulative reduction ratio in the unrecrystallized austenite region in low-alloy steel in the rolling step and the cooling rate for the steel in the next step are controlled to thereby make the steel have finer structure. Even in this, however, the ferrite grains formed could have a grain size of at least 10 microns in Si--Mn steel and a grain size of at least 5 microns in Nb steel. Thus, the technique is still limitative. On the other hand, as in Japanese Patent Publication (JP-B) 62-39228 and 62-7247, formed are ferrite grains having a grain size of around 3.about.4 microns or so by rolling steel under pressure to attain a total reduction ration of 75% or more at a temperature falling within a range of (Ar.sub.1 to Ar.sub.3 +100.degree. C.) including the two-phase range, followed by cooling it at a cooling rate of not lower than 20 K/s. As in JP-B Hei-5-65564, an extremely great reduction ration and a cooling rate of not lower than 40 K/s are needed for forming finer ferrite grains having a grain size of smaller than 3 microns. However, the rapid cooling at a rate of 20 K/s or larger is acceptable only in the production of steel sheets, but could not widely in the production of ordinary steel for weld constructions.
Given that situation in forming finer ferrite grains capable of contributing to the increase in the strength of steel, it is extremely difficult in the prior art to form finer ferrite grains having a grain size of smaller than 3 microns. In fact, no effective technique has heretofore been realized for forming such finer ferrite grains.
In addition, the increase in the reduction ratio in the unrecrystallized region in the controlled rolling causes another problem. For example, as in FIG. 11 (from "Iron and Steel", 65 (1979), 1747-1755), the increase in the working ratio results in the increase in the density of specific orientations (332) &lt;113&gt; and (113) &lt;110&gt; of ferrite grains, whereby the proportion of the small angle grain boundaries is increased. Even if fine grains having a grain size of 3 microns or so could be formed in steel, the strength and even the fatigue strength of the steel could not be increased so much to the level of the expected degree corresponding to the fined size of the grains. In addition, in that case, since there is a great probability that the ferrite grains formed all have the same orientation, large aggregates of the ferrite grains will grow. If so, it is essentially difficult to form fine ferrite grains. From this viewpoint, in the conventional technique of forming fine ferrite grains, the lowermost limit of the grain size is at least 5 .mu.m.
In the prior art, no technique was know at all for controlling the orientation of ferrite grains formed. Therefore, it was impossible to form fine ferrite grains while randomizing the orientation of the grains formed.